Became slower with cog switch
 

I rode 48x17 and my average speed for my daily 13.2 mile commute has always been above 18.4mph. Two weeks ago I switched to 48x19 and now my average speed for the same commute is 15.2mph. I'm going to switch out tonight and run it tomorrow and see the difference. Anyone else experience this & maybe an explaination? My logic was, lower GI = more cadence = higher speeds in the longrun. I guess I was wrong. Spinning @ 115rpm is too easy with 19t.

The "lower gear + higher cadence = more speed" thing works for some people, but others are just better suited to using larger gears. Your logic on using a lower gear to get comfortable with higher cadences is sound though, it just seems you're undergeared now. I'd use that as a training tool, and then apply the spinning ability the lower gear gives you to your higher normal gear.

It wasn't an issue for me but I dropped from low 80's to low 70s, 66 is scary easy in my opinion

By my calculation, 115 RPM in a 48 x 19 is over 23 mph. That assumes a 27" outer tire diameter, which is slightly less for a typical 700C x 25 clincher. At 15.2 mph you would be turning the cranks at only 75 rpm. At 18.4 mph in a 48 x 17 you would be turning the cranks at about 82 rpm, which is slightly faster in a higher gear. In any case you are not turning the cranks very fast on average, but maybe you meant that 115 RPM was your comfortable maximum. Are you using any foot retention, and if so what type? I use clipless pedals and run a 40 x 17 (64 gear inches), but I am a long time track racer and am very comfortable averaging over 100 RPM, and maxing out on the road at over 160 RPM (over 200 RPM on the rollers). In any case, I'm just making some observations and not trying to give you any advice, as gear selection is a personal thing and if you feel more comfortable with the bigger gear then just go for it.

So, is your cadence higher with the new gearing? If not, the lower gear will mean lower speed.

An explanation? You're forced to spin faster but your legs don't know how to yet.

Yes 115 seems to be my maximum as it tires me out quickly (within 5 minutes). The high 80s and 90s seems to be all the difference (can maintain that indefinitely).

I used to occasionally switch from 48x17 to 48x15. I can hit significantly higher top speeds and higher cruising speeds on the 15, but my average speed (over hundreds of miles) will still drop considerably. This I attribute both to the time it takes me to get up to speed, much slower speed climbing and when riding in traffic/stoplights/decents I have to play it a lot safer as it's harder to stop.

Assuming you commute on roads, your average speed of 18.4 is pretty good! My computer has about 5000 miles logged onto it (just bought it last winter) and I'm not sure of the breakdown of MUP miles vs Road miles, but even while averaging between 20mph (long MUP rides) and 24 mph (1-2 hour MUP rides) I can't get my overall average speed over 16.5. All the stopping/starting and riding carefully in traffic completely destroys my high speed average.